问题
I'm struggling a bit with async/await and returning a value from a Promise.
function test () {
return new Promise((resolve, reject) => {
resolve('Hello')
})
}
async function c() {
await test()
}
As I understood things I should be able to get a value by doing:
console.log(c())
But clearly I am missing a point here as this returns a promise. Shouldn't it print "hello"? On a similar note I am unclear as to whether a callback needs to be converted to a promise before wrapping it in async/await?
回答1:
I am missing a point here as this returns a promise. Shouldn't
console.log(c())
print "hello"?
No, async
functions always return promises. They're not magically running asynchronous code synchronously - rather the reverse, they turn synchronous-looking code (albeit speckled with await
keywords) into asynchronously running one.
You can get the result value inside the asynchronous function:
async function c() {
const result = await test()
console.log(result);
return 'World';
}
c().then(console.log);
I am unclear as to whether a callback needs to be converted to a promise before wrapping it in async/await?
Yes, you can await
only promises. See How do I convert an existing callback API to promises? for how to do the conversion.
回答2:
Async functions return a Promise. If the function throws an error, the Promise will be rejected. If the function returns a value, the Promise will be resolved.
来源:https://stackoverflow.com/questions/38839130/wrapping-promise-in-async-await